This invention relates to a process and catalyst for the dehydrogenation of hydrocarbons. More particularly, this invention relates to a catalyst and process for the dehydrogenation of paraffinic hydrocarbon utilizing a catalyst comprising a platinum group metal and zinc on a support component comprising L-zeolite and an alkali metal, useful for the production of high oxygen-content fuels blending components (Oxygenates) and chemical industry feedstocks.
Oxygenates have been part of the United States gasoline strategy since the late 1970s. With the recent enactment of the Clean Air Act Amendments of 1990, the demand for oxygenates has increased again such that gasoline is now being blended to 2.7 weight percent oxygen and is being marketed in numerous metropolitan areas that have failed to meet carbon monoxide pollution standards. In the near future, it is expected that between 30 and 60 percent of the United States gasoline pool may require oxygenates.
The most commonly used oxygenates today are methanol, ethanol, and ethers such as methyl tertiary butyl ether (MTBE). Although methanol and ethanol have high blending octanes, problems with toxicity, water miscibility, high Reid Vapor Pressure (RVP), high nitrogen oxide emissions, lower fuel efficiency, and cost have dampened industry enthusiasm for these components. As a result of the above, MTBE has become particularly attractive.
Homologues of MTBE such as ethyl tertiary butyl ether (ETBE) and methyl tertiary amyl ether (TAME) are also gaining industry acceptance. Moreover, commercial activity with respect to ETBE and TAME is expected to increase relative to MTBE, in view of recent Environmental Protection Agency decisions to reduce the RVP requirements for gasolines well below 9 psia, the blending RVP of MTBE.
Ether production capacity, however, is often limited by iso-olefin feedstock availability. Commercial MTBE and ETBE processes both utilize isobutylene as a feedstock while TAME processes utilize isoamylene as a feedstock. Isobutylene and isoamylene are generally supplied to a commercial ether process from a fluid catalytic cracking unit (FCC), a fluidized or delayed coker, or from downstream paraffin isomerization and dehydrogenation facilities, As a result, the availability of hydrocarbons having 4 or 5 carbon atoms is limited by constraints such as, but not limited to, crude properties, FCC catalyst properties and operating conditions, coking conditions, as well as by other refinery operating constraints. The chemical mix of C.sub.4 and C.sub.5 paraffins, olefins, and aromatics as well as the particular mix of iso-olefins to normal olefins are similarly constrained.
The relatively high ratio of capital and operating costs to the throughput of ether product subsequently produced from the construction of new facilities for increasing ether process feedstocks further exacerbates oxygenate supply. These costs are generally attributed to the high degree of complexity and the sophisticated equipment connected to the operation of dehydrogenation or isomedzation processes such as, but not limited to, desulfurization, catalytic reactor, and hydrogen supply and recirculation systems. The profitability of such new facilities is often dependent on the ability of the refiner to keep construction costs low and operating throughput high.
Thus, there exists a great need in the petroleum industry for a low cost method of increasing oxygenate production feedstocks that overcomes or avoids the obstacles described above and that is economically viable in terms of construction cost and facility utilization.
Processes for dehydrogenating paraffins in the presence of hydrogen and a catalyst comprising a platinum group metal on an amorphous alumina support have been disclosed in the art.
For example, U.S. Pat. Nos. 4,190,521, 4,374,046, and 4,458,098 to Antos disclose a catalyst comprising a platinum group component, nickel, and a zinc on a porous carrier material such as alumina for dehydrogenating paraffinic hydrocarbon.
U.S. Pat. No. 4,438,288 to Imai et al. discloses a dehydrogenation process using a catalyst comprising a platinum group component, an alkali or alkaline earth component, and optionally a Group IV component such as tin, on a porous support material such as alumina. The properties and characteristics of the catalyst generally necessitate periodic catalyst regeneration in the presence of a halogen.
Processes for dehydrogenating paraffins in the presence of hydrogen and a catalyst comprising a platinum group metal on a silicalite molecular sieve support have also been disclosed in the art.
For example, U.S. Pat. Nos. 4,665,267 and 4,795,732 to Barri and U.S. Pat. Nos. 5,208,201, and 5,126,502 to Barri et al. disclose processes for dehydrogenation of C.sub.2 to C.sub.10 paraffins using a catalyst comprising zinc and a platinum group metal on a support having the silicalite structure wherein the framework of the structure consists essentially of silicon and oxygen atoms or of silicon, zinc, and oxygen atoms. The catalyst is generally formed such that it is substantially free of all alkali or alkaline earth metals.
A process for dehydrogenating paraffins in the presence of a catalyst comprising a platinum group metal on an L zeolite molecular sieve support has also been disclosed in the art.
U.S. Pat. No. 4,727,216 to Miller discloses a process for dehydrogenating isobutane in the presence of a sulfur-containing gas and a dehydrogenation catalyst. The dehydrogenation catalyst comprises a sulfided L zeolite containing from 8-10% by weight barium, from 0.6-1.0% platinum, and tin at an atomic ratio with the platinum of about 1:1. The dehydrogenation catalyst further comprises an inorganic binder selected from the group consisting of silica, alumina, and aluminosilicates.
While the above-described processes and catalysts have achieved varying degrees of laboratory success, it has been found that in commercial application, catalysts such as those described above have been prone to early deactivation and short on-stream run lengths. In order to overcome this obstacle, the process operator has generally needed to perform the dehydrogenation reaction in the presence of supplemental hydrogen for reducing catalyst coke formation. Supplemental hydrogen supply facilities are particularly complex and generally require costly compression and hydrogen purification equipment. Moreover, supplemental hydrogen addition drives the dehydrogenation reaction stoichiometrically away from dehydrogenation and towards olefin saturation.
Notwithstanding the presence of supplemental hydrogen addition and recirculation equipment, the process operator has still generally needed to resort to catalyst regeneration. Catalyst regeneration is generally performed in large, high temperature catalyst regenerators present in fluidized bed/riser schemes or through periodic and frequent batch regeneration such as performed with semi-regenerative fixed bed schemes. Catalyst regeneration facilities are extremely costly. For fixed bed reaction schemes, an additional swing reactor must be erected and the process operated with at least one reactor off-stream and in regeneration mode all of the time.
It has now been found that a dehydrogenation catalyst comprising a platinum group metal and zinc on a support comprising L zeolite and an alkali metal provides superior dehydrogenation performance in terms of paraffin conversion, olefin selectivity, and olefin yield to that of the prior art dehydrogenation catalysts and maintains such level of superior performance, without regeneration, far longer than any of the prior art catalysts tested to date.
It has also been found that a dehydrogenation catalyst comprising a platinum group metal and zinc on a support comprising L zeolite and an alkali metal provides such an extended operating cycle life, that it can be used with or without supplemental hydrogen addition while still achieving superior levels of performance.
For purposes of the present invention, paraffin conversion, olefin selectivity, and olefin yield shall have the following meanings and shall be calculated by mole and in accordance with the following models: ##EQU1##
It is therefore an object of the present invention to provide a dehydrogenation process and catalyst that effectively dehydrogenate paraffinic hydrocarbon.
It is another object of the present invention to provide a dehydrogenation catalyst that resists deactivation and prolongs catalyst cycle life under dehydrogenation conditions.
It is yet another object of the present invention to provide a dehydrogenation process and catalyst that can be effectively operated in the absence of supplemental hydrogen addition.
It is still another object of the present invention to provide a dehydrogenation process that, in view of its simplicity, can be adapted to utilize equipment from any of several existing petroleum refinery or chemical plant operating facilities.
Other objects appear herein.